opamp based envelope follower

[zach]

Hi,

I'm having some trouble doing something I thought would be very simple: adding an envelope follower to the "Son of Screamer" circuit.  I've implemented an LED envelope follower before with a 4049 IC and my understanding is you just wire the LED anode through a small resistor to + voltage and the amplified signal to the cathode.  However, connecting the cathode to the output of the 4558 opamp causes it to be lit all the time, irrespective of the input.  I tried every size of capacitor I have on my bench in between, but they all cause it to not light up at all.  Similarly, wiring in a pot allows me to vary the LED brightness, but at no resistance is it responsive to the input.  Any idea what I'm doing wrong here?

Thanks,
Zach

[Gurner]

Hi,

I'm having some trouble doing something I thought would be very simple: adding an envelope follower to the "Son of Screamer" circuit.  I've implemented an LED envelope follower before with a 4049 IC and my understanding is you just wire the LED anode through a small resistor to + voltage and the amplified signal to the cathode.  However, connecting the cathode to the output of the 4558 opamp causes it to be lit all the time, irrespective of the input.  I tried every size of capacitor I have on my bench in between, but they all cause it to not light up at all.  Similarly, wiring in a pot allows me to vary the LED brightness, but at no resistance is it responsive to the input.  Any idea what I'm doing wrong here?

Thanks,
Zach

Without more info about what you're connecting to the cathode of the LED, it's gonna be difficult to help you. Chances are your 4558 opamp output is DC biased at 1/2 VCC therefore your LED *is* going to be on all the time (ie your LED's anode is connected to VCC< it's cathode to 1/2 VCC - hey lights!). Can you post up a circuit?

[zach]

Here's the circuit: http://www.beavisaudio.com/bboard/projects/bbp_BoutiqueTubeScreamer_Rev1_1.pdf

I'm wiring the envelope follower to the same spot as the output jack. 

[PRR]

> I've implemented an LED envelope follower before with a 4049

I can not picture that.

> LED anode through a small resistor to + voltage and the amplified signal to the cathode

Lit all the time. The _average_ voltage is always 4.5V; diode drop and resistor size set the current.

If you hit it with SUB-sonic signal (if the amp is rigged to pass subsonics), you will see the LED go bright and dark on each cycle. Though with the D1-D3 clippers in there, the change may be quite slight.

What do you want? A control voltage proportional to signal level? Or just an LED that lights proportional to signal?

FWIW: if the clipper is doing its job, the output level is "the same all the time"; that's what a clipper does. Changes of input level change the relative overtones and the ear is impressed, but a simple level sensor won't show much change.

[zach]

Sorry, meant to say 4093.  I understand what you're saying now about the average voltage always being 4.5V.  What I'm looking for is an LED that lights proportional to the amplitude of the signal.  Basically, I'm going to use it so a microcontroller can detect the attack via a vactrol.  The LED I want to light is not one of the clipping diodes, but rather wired in the same spot as the output jack. 

Thanks,
Zach

[ashcat_lt]

If you're putting it at the output jack (after the volume control?) then there shouldn't be any DC to worry about.  Unfortunately, as mentioned, the clipping diodes make it so that there isn't really any change in signal level.  If you get it to work, it'll indicate nothing but the volume control setting.  Won't it also clip the output?

[zach]

I actually removed the volume control, but yeah that's where it's at.  The LED is wired in parallel with the output jack, so it should not clip the output.  My understanding is that the clipping diodes will only, well, clip the signal.  If the signal in the input is below the clipping threshold, shouldn't it remain the same?  Therefore, if there's no signal going in, the LED is off?  I could also put the envelope follower after the first stage of the 4558, before the clipping diodes, but not sure the result would be any different. 

[Mark Hammer]

As noted, there needs to actually be dynamic variation for an envelope follower to have anything to "follow".  Certainly one of the things the TS topology is noted for is that it compresses the bejeezus out of the signal.  So sticking an envelope follower after it doesn't seem to serve any purpose that I can think of.

You're going to need to articulate your intentions and over-arching goals a little better here.  I'm not saying it's a stupid idea.  I'm just having difficulty imagining any practical outcome of what has been described so far in this thread. 

[zach]

As I said, the point is for a microcontroller to detect the attack via a vactrol.  In other words, you pick a string and the microcontroller can tell.  Also as mentioned, I've done this before with a 4093 and had no problems.  Furthermore, as I said below, if the clipping presents a problem, I can always insert the envelope follower before the clipping diodes after the first stage of the 4558. 

[Mark Hammer]

Yeah, I suspect that the best place to tap the signal for detecting the envelope would be from the output of the transistor input buffer stage.  At that point, it's an unconstrained full-bandwidth signal.

[zach]

Hmm, I'm seeing the same behavior before the clipping diodes.  I didn't think they were the problem anyway since I'm using germanium ones and am only interested in the attack anyway. 

Even 68pf cap between the output of the 4558 and the cathode of the LED causes it to not light up no matter how loud.  I really must not be doing something right here.